The present invention relates to a method and apparatus for the desalination of Sea Water (SW) or Brackish Water (BW) by Reverse Osmosis (SWRO or BWRO) in closed circuit.
Desalination by reverse osmosis (RO) occurs when salt water solution is compressed against semi-permeable membranes at pressure higher than its osmotic pressure. The “Plug Flow Desalination” (henceforth “PFD”) method involves the passing of pressurized feed flow through pressure vessels (modules) with semi-permeable membrane elements, wherein it separates continuously into non-pressurized flow of desalted permeate and pressurized flow of brine effluent. The SWRO or BWRO recovery of PFD relates to the feed concentration, the applied pressure, the number of elements per module, the staging of modules and the specifications of elements. Pressurized feed flow in PFD is always greater than the permeate flow and 50% recovery implies that pressurized feed flow is twice that of permeate. PFD may also involve partial recycling to enhance recovery and this method is characterized by the continuous flow of its three components; the pressurized feed, the pressurized brine effluent and the permeate.
In contrast with PFD, Closed Circuit Desalination (henceforth “CCD”) is a batch process performed with solution recycling under hydrostatic pressure with same rate of pressurized feed supply and permeate delivery. Internal flow in CCD is created by circulation means in order to minimize concentration polarization effects. The CCD technology was first reported in the late eighties by Szucz et al. in U.S. Pat. No. 4,983,301 and by Bratt in U.S. Pat. No. 4,814,086 and the making of such a such batch process continuous was proposed by means of “ . . . two relatively large capacity tanks . . . ” that are alternately engaged in the closed circuit, thereby, allow the recharge of the closed circuit with fresh feed without the stopping desalination. Earlier patents from the late seventies, such as DE 26 22 461 A (1977) and JP 54 128985 A (1979), describe PFD with partial re-circulation that is fundamentally different from CCD in spite of the similarity of certain fixtures (e.g. circuit with membrane module(s), lines for feed and for recycling, circulation means, pressurizing means, means for effluent discharge, monitoring means, etc.). The distinction between CCD and PLD is clear and unmistakable, since only the former process involves the replacement of brine effluent with fresh feed at atmospheric pressure, thereby avoiding the need for energy recovery from the pressurized brine effluent that is essential in the latter process.                Specification, page 2, line 26, “consecutive” has been replaced “consecutive”.        
In comparison with PLD, the CCD requires much smaller means for pressurization fresh feed and proceeds with lower specific energy without need of energy recovery. The CCD technology according to said patents is a batch process made to operate continuously by means of consecutive sequential steps involving two relatively large capacity containers and this approach has not been considered economically favored for the design of desalination plants.
The present invention describes both apparatus and methods whereby Closed Circuit Desalination (CCD) may be performed continuously by means of a relatively small single container, thereby, the making of this technology economical feasible for diverse applications involving both sea water and/or brackish water desalination.